Method for forming a protective coating with enhanced adhesion between layers

ABSTRACT

A method for forming a protective coating on a substrate comprising, applying a bond coating to the substrate, the bond coating having a first surface roughness, ionizing an inert gas which flows into the surface of the bond coating so as to impart a second surface roughness to the bond coating greater than the first surface roughness, wherein the inert gas is ionized and caused to flow into the surface of the bond coating by a reverse polarity current supplied to an electrode which removes at least one electron from the inert gas, and applying a top coating to the bond coating. Additionally, a method for preparing a surface to receive and adhere to a coating comprising roughening the surface to create a micro-roughening network on the surface. In addition, a method of improving strain tolerance and cyclic spallation life of a protective coating.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.11/307,266 filed on Jan. 30, 2006, the disclosure of which isincorporated herein by reference.

TECHNICAL FIELD

This invention relates to protective coatings and methods for formingthe same.

BACKGROUND OF THE INVENTION

Coatings are often applied to metallic surfaces to protect against wear,erosion, corrosion, oxidation or to lower surface temperatures.Coatings, such as oxidation-corrosion protection coatings for a metal,function by diffusing protective oxide forming elements like aluminumand chrome to the surface that is exposed to harmful externalities.Thermal barrier coatings (TBCs) are made up of a bond coating on thesubstrate and a top coating on the bond coating. Examples of bondcoatings include diffusion aluminide bond coatings. The top coating istypically zirconia based and may comprise yttria, magnesia, ceria,scandia or rare earth oxide partially stabilized zirconia.

Application of these protective high temperature oxidation coatings canbe by thermal spray and diffusion techniques. The top coating may beapplied air plasma spray (APS) or electron beam physical vapordeposition (EB-PVD). EB-PVD has been used successfully in commercialapplications of ceramic top coatings to aluminide diffusion bondcoatings to create TBCs that are strain tolerant and have goodspallation life for high thermal cycle applications. Likewise,application of top coatings using APS has been found to createmicrostructures with vertical cracks that improve TBC cyclic spallationlife. However, attempts to apply this air plasma spray, dense verticallycracked (DVC) top coating to aluminide bond coatings have beenunsuccessful due to lack of adhesion to the smooth surface of the bondcoatings. In cases where DVC top coatings have adhered to a bondcoating, the spallation life of the TBCs have been inferior to TBCs withbond coatings having two to three times the surface roughness.

Accordingly, there is a need for a simple and economically desirablemethod for preparing a bond coating surface to receive and adhere to atop coating for TBCs with improved strain tolerance and cyclicspallation life.

SUMMARY OF THE INVENTION

This disclosure addresses the above described need in the art byproviding a method for forming a protective coating on a substratecomprising, applying a bond coating having a first surface roughness,ionizing an inert gas which flows into the surface of the bond coatingso as to impart a second surface roughness to the bond coating greaterthan the first surface roughness, and applying a top coating to the bondcoating. The inert gas is ionized and caused to flow into the surface ofthe bond coating by a reverse polarity current supplied to an electrodewhich removes at least one electron from the inert gas. The positivelycharged ions of the inert gas are repelled by the positively chargedelectrode and flow into surface of the bonding agent, causingparticulate fragments of the surface of the bond coating to break off.Therefore, the ions create microscopic craters in the surface of thebonding agent. Consequently, this roughening of the surface of the bondcoating improves the adherence of the top coating to the bond coating.

Other objects, features, and advantages of this invention will beapparent from the following detailed description, drawing, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-C show a schematic of a method for forming a thermal barriercoating on a substrate in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF EMBODIMENTS

As summarized above, this disclosure encompasses a method for forming aprotective coating on a substrate, a method for preparing a surface toreceive and adhere to a coating. In a particular embodiment, a methodfor improving the strain tolerance and the cyclic spallation life of athermal barrier coating (TBC) is disclosed. Embodiments of thisinvention are described in detail below and illustrated in FIGS. 1A-C.

A thermal barrier coating (TBC) 10 formed on a substrate 12 by a methodin accordance with an embodiment of this invention is illustrated inFIG. 1C. The TBC 10 comprises a bond coating 14 and a top coating 16.Although this embodiment illustrates a TBC, it should be understood thatthis invention is applicable to other types of coatings.

As show in FIG. 1A, the bond coating 14 is applied to the substrate 12.The substrate can comprise, but is not limited to, any nickel or cobaltbased alloy. For example, the substrate may comprise a superalloy suchas GTD-222 (51Ni19Co22Cr1.2Al2.3Ti.94Ta.8Nb2WCBZr). The bond coating 14may be applied using various methods, including high velocity oxy-fuelspraying. Suitable materials for use as a bond coating 14 include, butare not limited to, aluminide diffusion bond coatings. These aluminidediffusion bond coatings may include modified or alloyed aluminides,chromium aluminide (CrAl), palladium aluminide (PdAl), platinumaluminide (PtAl), silicon modified aluminides, simple aluminide, andover aluminized MCrAlY, where M stands for Fe, Ni, Co, Si, Hf, Ta, Re,noble metals, or a mixture of Ni and Co or additional elements andcombinations that well known to those skilled in the art. Additionally,aluminide diffusion bond coatings may be about 1 mil to about 4 milsthick.

The surface of the bond coating 14 as applied to the substrate 12 has afirst roughness that is inherently smooth. For example, a bond coating14 made of aluminide has a surface roughness of less than about 60microinches Ra, where Ra is the arithmetic mean of displacement valuesas calculated to quantify the degree of roughness achieved. The inherentsmoothness of the bond coating 14 results in poor adherence of a topcoating 16, particularly air plasma spray (APS) top coatings.Consequently, the bond coating 14 is roughened to improve adherence ofthe top coating 16 to the bond coating.

As shown in FIG. 1B, a micro-roughening network 18 is created on thesurface of the bond coating 14 by using an electrode 22 to ionize aninert gas and cause the ions 20 to flow into the bond coating surface.To ionize the inert gas, the electrode 22 is supplied a reverse polaritycurrent (not shown). This reverse polarity current is a direct currentset at a high frequency to create the ions 20 in the inert gas. Thereverse polarity current is also set at an amperage between about 0 andabout 10 amperes. A higher amperage setting results in a roughnessgreater than a roughness that would result from a lower amperagesetting. Once the electrode 22 is supplied a reverse polarity current,it removes at least one electron from the inert gas that is suppliedadjacent to the bond coating 14. The inert gas may be, but is notlimited to argon. While argon may be used as the inert gas, it should beunderstood that any inert gas may be used, provided that it is may beionized and used in roughening the bond coating 14 in accordance withthe methods of the present invention. As a result of the removal of atleast one electron, the inert gas is ionized to a positive charge andthe positively charged electrode 22 repels the ions 20 toward the bondcoating 14. These ions bombard the bond coating 14, causing particulatefragments to break off and microscopic craters to form. Thus, theionized inert gas 20 imparts a second surface roughness to the bondcoating 14 greater than the first surface roughness.

The second surface roughness of the bond coating 14 may be between about75 microinches Ra to about 750 microinches Ra. More particularly, thesecond surface roughness of the bond coating 14 may be between about 100microinches Ra to about 600 microinches Ra. Still more particularly, thesecond surface roughness of the bond coating may be between about 150microinches Ra to about 450 microinches Ra. This second surfaceroughness resulting from the creation of the micro-roughening network 18on the bond coating 14 promotes adhesion and mechanical bonding of thetop coating 16 to the bond coating.

The roughening of the bond coating 14 to create the micro-rougheningnetwork 18 may be manual or automated using a mechanical device such asa robot. In addition, the bond coating 14 may be roughened in multiplepasses to impart the desired second surface roughness.

The ionizing of the inert gas may be accomplished by using a reversetransfer arc welding torch. The reverse transfer arc welding torch maybe a gas tungsten welding torch, a plasma arc welding torch, or any arcwelding torch with a plasma source. Although a reverse transfer arcwelding torch may be used in the present invention to ionize the inertgas, it should be understood that an electric arc is not conducted fromthe electrode in the reverse transfer arc welding torch to the bondcoating. The formation of an electric arc between the electrode 22 andthe bond coating 14 may melt the bond coating or cause cracking in thebond coating. To prevent the formation of an electric arc, the electrodeis positioned at least about three times further from the bond coatingthan the distance the electrode would be positioned for arc welding. Forexample, a gas tungsten welding torch is positioned about 0.5 inches toabout 1 inch away from a surface to be welded. In contrast, a gastungsten welding torch used in a method in accordance with the presentinvention is positioned about 1.5 inches to about 3 inches from the bondcoating to prevent an electric arc from forming.

In addition, the ions 20 which roughen the surface of the bond coating14 bombard the bond coating at a slow speed relative to the speed atwhich the electrons strike the electrode. Consequently, only smallamounts of heat are carried to the bond coating 14. Conversely, theelectrons strike the electrode 22 at a high velocity and carry asubstantial amount of welding heat. This, the heat may be removed fromthe electrode by water-cooling, for example.

Once the micro-roughening network 18 is created on the bond coating 14,the top coating 16 may be applied to the bond coating as shown in FIG.1C. Adhesion and mechanical bonding of the top coating 16 to the bondcoating 14 is improved by the micro-roughening network 18. The topcoating 16 may be applied by air plasma spray (APS), for example. APS isparticularly suitable for application of a dense vertically cracked(DVC) top coating 16. This DVC top coating 16 has vertical cracks withinthe top coating that consequently improve the TBC strain tolerance andcyclic spallation life. Suitable materials for use as the top coating 16include, but are not limited to, ceramic materials. These ceramicmaterials may comprise yttria, magnesia, ceria, scandia or rare earthoxide partially stabilized zirconia. For example, the top coat maycomprise yttria stabilized zirconia in an amount of 8% by weight of thetop coat. In addition, the top coating 16 may be about 10 mils to about100 mils thick.

The methods of forming TBCs of this invention may be used in articleshaving a TBC. Examples of such articles include a gas turbine or adiesel engine. In addition, the embodiments of the TBC may be formed onnickel or cobalt based alloys.

The present invention is further illustrated below in an example whichis not to be construed in any way as imposing limitations upon the scopeof the invention. On the contrary, it is to be clearly understood thatresort may be had to various other embodiments, modifications, andequivalents thereof which, after reading the description therein, maysuggest themselves to those skilled in the art without departing fromthe scope of the invention and the appended claims.

EXAMPLE 1

An example of an embodiment of a method for forming a TBC is disclosedin this example. General techniques of forming a TBC are well known inthe art and are disclosed, for example, in U.S. Pat. No. 5,830,586, thedisclosure of which is expressly incorporated herein by reference in itsentirety.

In this embodiment, the forming of the TBC comprises applying analuminide diffusion bond coating to either a nickel or cobalt basedsuperalloy substrate. This bond coating has a smooth surface which isnot optimal for applying an air plasma sprayed top coat. Thus, inert gasargon is then ionized by a gas tungsten arc welding machine and used toroughen the surface of the bond coating. The electrode is positioned ata distance from the aluminide diffusion bond coating to insure that anelectric arc does not form. The reverse polarity current then removeselectrons from the argon and creates positively charged argon ions whichare repelled by the positively charged electrode towards the aluminidediffusion bond coating. The gas tungsten arc welding machine istraversed at a rate of about 1 inch per minute to impart a surfaceroughness of 150 microinches Ra to about 450 microinches Ra onto thebond coating. A top coating is air plasma sprayed onto themicro-roughening network created on the bond coating. The air plasmaspraying of the dense vertically cracked top coating improves straintolerance and cyclic spallation life of the TBC.

It should be understood that the foregoing relates to particularembodiments of the present invention, and that numerous changes may bemade therein without departing from the scope of the invention asdefined from the following claims.

We claim:
 1. A substrate having a protective coating comprising: a bondcoating attached to the substrate, the bond coating having a firstsurface roughness; a micro-roughening network on a surface of the bondcoating; a top coating adhered to the micro-roughening network; whereinthe micro-roughening network has a second surface roughness greater thanthe first surface roughness and is obtained when an inert gas is ionizedand caused to flow into the surface of the bond coating by a reversepolarity current supplied by an electrode that removes at least oneelectron from the inert gas.
 2. The substrate of claim 1, wherein theprotective coating further comprises a thermal barrier coating.
 3. Thesubstrate of claim 1, wherein the inert gas is ionized using a reversetransfer arc welding torch.
 4. The substrate of claim 1, wherein thefirst surface roughness is less than about 60 microinches Ra.
 5. Thesubstrate of claim 1, wherein the second surface roughness is betweenabout 75 microinches Ra to about 750 microinches Ra.
 6. The substrate ofclaim 1, wherein the second surface roughness is between about 100microinches Ra to about 600 microinches Ra.
 7. The substrate of claim 1,wherein the electrode and the bond coating are devoid of an electric arcbetween each other.
 8. The substrate of claim 1, wherein the bondcoating is an aluminide diffusion bond coating.
 9. The substrate ofclaim 8, wherein the aluminide diffusion bond coating comprises a bondcoating material selected from the group consisting of modified oralloyed aluminides, CrAl, PdAl, PtAl, simple aluminide, silicon modifiedaluminides, and over aluminized MCrAlY.
 10. The substrate of claim 1,wherein the top coating comprises a ceramic material.
 11. The substrateof claim 10, wherein the ceramic material is selected from the groupconsisting of yttria, magnesia, ceria, scandia, and rare earth oxidepartially stabilized zirconia.
 12. The substrate of claim 1, wherein thetop coating is a dense vertically cracked coating.
 13. A substratehaving a protective coating with improved strain tolerance and cyclicspallation life comprising: a bond coating attached to the substrate; amicro-roughening network on a surface of the bond coating; and a topcoating adhered to the micro-roughening network; wherein themicro-roughening network is obtained when an inert gas is ionized andcaused to flow into the surface of the bond coating by a reversepolarity current supplied by an electrode that removes at least oneelectron from the inert gas.
 14. The substrate of claim 13, wherein theionized inert gas that flows into the surface of the bond coatingroughens the surface of the bond coating.
 15. The substrate of claim 14,wherein the surface of the bond coating has a roughness that is betweenabout 75 microinches Ra to about 750 microinches Ra.
 16. A protectivecoating comprising: a bond coating having a first surface roughness; anda top coating adhered to the bond coating; and a micro-rougheningnetwork surface at an interface of the bond coating and the top coating,wherein the micro-roughening network surface has a second surfaceroughness greater than the first surface roughness; wherein themicro-roughening network surface is obtained when an inert gas isionized and caused to flow into a surface of the bond coating by areverse polarity current supplied by an electrode that removes at leastone electron from the inert gas.
 17. The protective coating of claim 16,wherein the bond coating comprises a bond coating material selected fromthe group consisting of modified or alloyed aluminides, CrAl, PdAl,PtAl, simple aluminide, silicon modified aluminides, and over aluminizedMCrAlY.
 18. The protective coating of claim 16, wherein the top coatingcomprises a ceramic material selected from the group consisting ofyttria, magnesia, ceria, scandia, and rare earth oxide partiallystabilized zirconia.
 19. The protective coating of claim 16, wherein thesecond surface roughness is between about 75 microinches Ra to about 750microinches Ra.
 20. The protective coating of claim 19, wherein thesecond surface roughness is between about 150 microinches Ra to about450 microinches Ra.